# JavaScript: Generate an array, containing the Fibonacci sequence, up until the nth term

## JavaScript fundamental (ES6 Syntax): Exercise-237 with Solution

Write a JavaScript program to generate an array, containing the Fibonacci sequence, up until the nth term.

• Use Array.from() to create an empty array of the specific length, initializing the first two values (0 and 1).
• Use Array.prototype.reduce() and Array.prototype.concat() to add values into the array, using the sum of the last two values, except for the first two.

Sample Solution:

JavaScript Code:

``````//#Source https://bit.ly/2neWfJ2
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);
console.log(fibonacci(2));
console.log(fibonacci(3));
console.log(fibonacci(6));
```
```

Sample Output:

```[0,1]
[0,1,1]
[0,1,1,2,3,5]
```

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-237-1 by w3resource (@w3resource) on CodePen.

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## JavaScript: Tips of the Day

Checking if a key exists in a JavaScript object?

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

```var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!
```

You should instead use the in operator:

`"key" in obj // true, regardless of the actual value`

If you want to check if a key doesn't exist, remember to use parenthesis:

```!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"
```

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

```obj.hasOwnProperty("key") // true
```

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined? var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists! You should instead use the in operator: "key" in obj // true, regardless of the actual value If you want to check if a key doesn't exist, remember to use parenthesis: !("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj" Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty: obj.hasOwnProperty("key") // true For performance comparison between the methods that are in, hasOwnProperty and key is undefined.

Ref: https://bit.ly/2CFNp1X